Resistance material



Oct. 14, 1941.

R. o. GRISDALE 2,258,646

RES I STANCE MATERIAL Filed May 1'7. 1939 SPECIFIC RES/STANCE.

I06 FIG. 3

o z a 4 a 6- 7 a 9 ATOMIC RATIO OF ME TALS lNl/ENTOR R0. GR/SDALE ATTORNEY for regulation and li Patented Oct. 14, 1941 UNITED ,STATES' PATENT \OFFICE RESISTANCE MATERIAL Richard 0. Grisdale,-Elmsford, N. Y., assignor to Bell Telephone Laboratories, Incorporated,

New

York, N. Y., a corporation of New York Application May 17, 1939, Serial No. 274,114

12 Claims. (01. 201-76) This invention relates to electrical resistor ma--;*

is generally too low and that of insulators too high to make them of value as circuit elements The so-called semiconductors, which have intermediate resistance values, therefore, appear to comprise the useful field of materials having relatively high resistance-temperature coeificients.

Several of the semiconducting materials, ineluding oxides of metallic elements, have been employed heretofore for negative resistance-temperature coefficient resistor units. Although such units have in some cases been reasonably satisfactory from a performance viewpoint, they are rather difficult and expensive to prepare and he purposes.

' maintain.

One object of this invention is, therefore, to improve resistor materials whose resistances are highly dependent upon temperature.

Another object of this invention is to produce satisfactory resistor devices having high resistance-temperature coefficients from materials which are relatively inexpensive compared with those previously used with success.

In accordance with one feature of this inven tion, the resistor devices comprise combinations of oxides of metallic elements.

In accordancewith another feature of the invention, the metal oxides employed are of such character that when intimately mixed and heated they combine to form a compound, solid solution, eutectic mixture, or some combination of these.

A further feature of this invention resides in the characteristic internal structure of the resistance material made in accordance therewith, in that it includes a crystal structure uniquely different from that of the materials from which it is made.

Another feature of this invention resides in the use, as resistance materials, of some combinations of metal oxides that exhibit a minimum resistance when the ratio of the numbers of the atoms of the metals present is that of integers excess of one metal over the amount required by an integral atomic ratio Still another feature of this invention. comprises the making of resistance units with negative temperature coefficients of resistance from oxides of manganese and iron or oxides of nickel and manganese, by heat treatment to form resistor units containing compounds of said oxides such as manganese ferrite and nickel manganite.

- The invention and the foregoing and other features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which:

1 is a sectional view of a disctype resistor unit illustrative of one embodiment of this invention;

Fig. 2 is a sectional view of a bead type resistor unit illustrative of another embodiment of this invention; and

Fig. 3 is a graphical representation of data showing the relation between specific resistance and atomic ratio of the metallic elements, for a series of resistors made from different proportions of iron and manganese oxides and different proportions of nickel and manganese oxides.

It has been found that, where there is a tendency towards reaction or solid solution of two or more metallic oxides, semiconduction, which is predominantly electronic, can be expected, the

ture of the atmosphere in which the heat treatment is carried out. Actually, alterations in heat treatment and atmosphere displace the chemical equilibrium between the components and hence change the composition of the reaction product. There are, however, two types of alteration in composition, each of which has a large efiect on the specific resistance of an oxidic material. It has been shown theoretically and demonstrated experimentally that very slight departures from the stoichiometrical composition in an oxide system can produce very large changes in its conductivity. Thus, an excess of oxygen or metal over that required by the chemical formula can bring about changes in conductivity which are dependent on the concentration of the over andwhich have a higher resistance if there is an abundant element.

The stability or constancy of composition an oxide indicates that the element in excess is of two natures. They may consist of a solution I of the new compound in one or more of the intial firmly bound, either by secondary valence forcesmobile or chemically reactive. Thus, the dependence of the conductivity on the ambient oxygen concentration is determined by preparing the samples at high temperatures in various atmospheres and subsequently measuring their conductivities at low temperatures where they are stable over long periods of time regardless of the atmosphere. It is only after these specimens are again heated at high temperatures that the resistances are altered. This type of change is dependent upon the mobility of the constituent atoms at high temperatures and since their mobilities are very small at normal temperatures the excess atoms are frozen" firmly in place. For an oxidic material given. suitable heat treatment, the ratio of combined to uncombined elements is therefore constant at normal temperatures. There is for each temperature and each atmosphere a definite composition and any change in environment results in a tendency for the system to alter its composition. However,

for most oxides at normal temperature the rate of approach to a new equilibrium is so small that apparent constancy of composition is observed.

While the ratio of combined to uncombined atoms for an oxidic system is a constant fora specific heat treatment, it has been determined that in some systems the specific resistance is further dependent upon the ratio of the number of .atoms of one metal to that of another. When this ratio has a value equal to the ratio of two integers, the two metals will be present in the proper proportions to form a compound. When no oxygen is lost from or taken up by the oxides during heat treatment the compound is simply the addition product of the two.

This need not be the case, however, and the same compounds maybe formed from oxides in other valence states with subsequent loss or absorption of oxygen. Thus, the fact that the specific resistance of the reaction product is dependent only upon the atomic ratio of the two metals and not on their valences in the original oxides,

indicates that for the heat treatment employed only one oxide of each metal is stable, or that the. compound formed is more stable than any oxide of either metal. in certain oxide systems the specific resistance is a minimum when the atomic ratio has a value that is the ratio of two integers or verynearly so.

various metals present.

ingredients or they may be solutions of these initial ingredients in the new crystalline phase re- .sulting from the heat treatment.

The curves of Fig. 3 illustrate the foregoing statement with respect to minimum resistance.

Curve A, which is for a series of units having different proportions of iron and manganese oxides, exhibits a pronounced minimum for an atomic ratio Fe/Mn=2. The minima in'curves B and C, which are for combinations of manganese and nickel oxides, are not so sharply defined, occurring between Mn/Ni=2 and Mn/Ni=4. The differences between curves B and C are due to diflerences in heattreatment.

The specific resistance of a dense oxidic material is then dependent upon the ratio of combined to free elements and, in systems of two or more oxides, upon relative amounts of the In general, the resistancepof a semiconductor is dependent also upon its porosity, upon the size of the individual crystallites, and upon its homogeneity. As the porosity of an oxidic material decreases, its resistance also generally decreases .and in the limit approaches that of a completely dense mass. The stability of oxidic semiconduction increases with decrease in porosity because in a porous material the current is concentrated in small areas which in some cases are raised to temperatures sufiicient to alter the equilibrium composition. The effect of heating a porous semiconductor by a current flowing through it may then be a permanent change in its resistance. In general, the porosity of an oxidic system in which compounds, eutectoids and/or solid solutions are formed by heat treatment is less than It has been found that In some cases, the minimum in resistance as a function oi the atomic ratio of the metals is pronounced, occurring, at a value of this ratio corresponding to a definite oxidiccompound characteristically different from either of the original oxides. In other cases, the minima are less sharply defined, although they are associated with atomic ratios nearly the same as those corresponding to compound formation. In either case, it has been found by chemical and X-ray analysis that there is present in the material of lower specific resistance a solid phase having a crystal structure uniquely different from that of any of the initial ingredients. In those cases where the minima in resistance are sharply defined it is probable that there is no extensive solid solution. However, where a new crystal phase is present and where the minimum does not occur at an integral atomic ratio there are solid solutions present. These solutions may be ply by sintering and recrystallization.

As has been stated, the specific resistance and the mechanical properties of oxidic materials are highly dependent upon their composition. For qualitative purposes the specific resistance p of such materials may be given by the expression p=poe-a(T--To) where pc is the specific resistance at temperature. To, a the temperature coeflicient of resistance and T the temperature, and where p0 and a are constant. Actually a is a decreasing function of increasing temperature but for purposes of comparison it can be considered as constant over small temperature ranges. It has been observed in the study of some metallic oxides, where the resistance was varied by heat treatment, that a measured over a given range, increased as the resistance increased. It appears however, that this is not a universally valid rule,-for certain oxide systems have been found for which on within the temperature range used for comparison is substantially independent of the specific resistance p0 at the reference temperature To, even though [10 is highly dependent upon the atomic ratio of the metals in- I volved. It has been observed that p may be essolid solution, a eutectic mixture. or some combination of these. Moreover, as has been previously stated, the combined material includes a crystal structure that is characteristic and not like that of any of the constituents. "The new material may for purposes of this specification and the appended claims be called a homogene-.

- portions of the two oxides are such that there are two atoms of iron for each atom of manga nese, that is, for an atomic ratio Fe/Mn=2.0. the specific resistance is a minimum as illustrated by the curve A in Fig. 3. I

A resistance unit comprising nickel manganite made from nickel and manganese oxides heat treated at a temperature ranging from 1000 to 1450 C. shows like characteristics. As may be seen from curves B and C Fig. 3 the specific resistance is minimum between atomic ratios Mn/Ni=2.0 and 4.0.

Although in either case the variation in the specific resistance p for small changes in composition is large over a considerable range, the resistance-temperature coefficient a remains substantially constant. Other oxide combinations such as zinc and uranium or manganese and uranium also she a minimum of specific resistance where the metals are present in such quantities that their atomic ratio is nearly that of two integers.

shown in Fig. 1 is of the disc or plate type.-

The semiconducting material is denoted by I and the terminals or electrodes by ll. Units of this type may be,made by grinding and intimately mixing predetermined amounts of the constituent oxides. A die of proper dimensions may then be charged with the mixed oxides and pressure applied. A pressure of the order of ten tons per square inch is suitable for producing satisfactory discs or plates. The pressed discs are then heat treated to bring about the necessary combination of elements. Firing temperatures of from 450 to 1500 C. have been found suitable. The electrodes H may be formed by applying silver paste to appropriate surfaces of material I0 and heating to solidify the paste.

The atmosphere in which the units are heated may be adjusted in accordance with the oxides employed in order to obtain a desired final resistance. For example, with mixture of F620: and M11304 or N and M11304 the resistance of the resulting unit is least for a pure oxygen atmosphere and greatest for pure nitrogen. Air gives results intermediate these two extremes and atmospheres containing water vapor, illuminating gas or hydrogen give low resistances. When ance due to changes in atmosphere, are com pletely reversible.

The unit shown in Fig. 2 is of the bead type comprising a head I! of a selected semiconducting material with conducting leads l3 embedded therein. The leads I: should be of some refractory conducting material such as platinum;

In the manufacture of these units, the constituent oxides are ground and mixed as for the disc type devices. The mixed-oxides are made into a paste by the addition of a suitable liquid such as distilled water or a solution of a metal salt. The paste is formed into small beads on platinum or other suitable wires and dried. The units are then heat treated in a manner similar to that employed for disc units.

the metals are present in an atomic ratio of The conduction of oxide resistors, that are made in accordance with this invention, is predominately electronic, making them available for use with direct as well as alternating current. The devices are very stable at normal operating temperatures and the resistance-temperature cycle is completely reversible.

By employing the materials and following the procedure of the foregoing disclosure, resistances may be produced having a sufficiently high temperature coefficient of resistance to make them useful as circuit elements. By varying the heat treatment and composition of the oxide mixture, resistances may be prepared which have a wide range of values of specific resistance but all having essentially the same resistance-temperature coefilcient.

Although specific embodiments of this invention have been shown and described, it will be understood, of course, that they are but illustrative and that modifications may .be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

l. A resistance unit consisting of a compound of manganese oxide and an oxide from the group consisting of iron oxide and nickel oxide and an excess of one constituent oxide, there being solid solution between the compound and the excess oxide.

2. A resistance material consisting of a homogeneous combination of manganese oxide and an oxide from the group consisting of iron oxide and nickel oxide, and having a specific resistance dependent upon the atomic ratio of the constituent metals and a temperature coeificient of resistance substantially independent of said ratio.

3. A resistance material consisting of a homogeneous combination of manganese oxide and an oxide from the group consisting of iron oxide and oxide from the group consisting of iron oxide and nickel oxide and having a specific resistance dependent upon the atomic ratio of the metals in the combination, said specific resistance having a minimum value for an atomic ratio approximating that necessary for a compound between the oxides.

5. A resistance unit consisting of a compound of iron oxide and manganese oxide. I

6. A resistance unit consisting of a compound of manganese oxide and nickel oxide.

7. A resistance material consisting of a com-- bination of iron oxide and manganese oxide and 8. A resistance material consisting of a combination oi manganese-oxide and nickel oxide and having a specific resistance dependent upon the atomic ratio of the manganese "and nickel in the combination,- said specific resistance being a minimum for atomic ratios between 9. The method of making s resistance unit that comprises intimately mixing finely divided oxides of iron and manganese, forming said mixture into a body, and heat treating said body-at a temperature ranging from 1000 to 1450" C.

10. The method of making a resistance unit Qassasie that comprises intimately mixing finely fdlvlded oxides of manganese and nickel, forming said mixture into a body. andvheat treating said body at a temperature ranging from 1000 to 1450 0.

11. The method oi making a resistance unit that comprises intimately mixing finely divided manganese oxide and a finely divided oxide from the group consisting of iron oxide and nickel oxide, forming the mixtureinto a body and heat treating said body at a temperature ranging from 1000 C. to 1450 C.

12. A resistance unit consisting 0f manganese I the group consisting of oxide andan oxide from iron oxide and nickel oxide, and a compound of the constituent oxides having a specific resistance lower than that or either constituent oxide.

. RICHARD O. GRISDALE. 

